This application claims the benefit of a Japanese Patent Application No. 2001-216610 filed Jul. 17, 2001, in the Japanese Patent Office, the disclosure of which is hereby incorporated by reference.
1. Field of the Invention
The present invention generally relates to write pulse generators and optical disk units, and more particularly to a write pulse generator for generating write pulses in an optical disk unit employing a constant angular velocity (CAV) recording system and to an optical disk unit which uses such a write pulse generator.
2. Description of the Related Art
FIG. 1 is a diagram showing an optical disk 10L employing a constant linear velocity (CLV) recording system. In the case of the optical disk 10L, a channel bit period T which is used as a reference for the recording and reproduction is always constant. A recording pit pattern, which is described by a ratio to the channel bit, that is, an integer multiple of the channel bit, is also formed at a constant linear density.
However, in the optical disk 10L, a rotational speed (or a number of revolutions per unit time) must be changed depending on a radial position where the recording or reproducing operation is carried out. In other words, the rotational speed of the optical disk 10L must be finely controlled to become inversely proportional to the radial position where the recording or reproducing operation is carried out, so that the rotational speed is lower at an outer periphery of the optical disk 10L and is higher at an inner periphery of the optical disk 10L.
FIG. 2 is a diagram showing an optical disk 10A employing a CAV recording system. In the case of the optical disk 10A, the rotational speed (number of revolutions per unit time) is constant. However, the channel bit frequency must be set lower at the outer periphery of the optical disk 10A, and set higher at the inner periphery of the optical disk 10A.
In an optical disk unit which is loaded with the optical disk, such as the optical disk 10L or 10A, desired recording pit patterns are formed by irradiating a laser beam which is emitted from a laser diode onto the optical disk, so as to record information on the optical disk.
When irradiating the laser beam on the optical disk, it is known that the duration of the laser beam emission needs to be finely adjusted, by taking into consideration the characteristic of the optical disk itself, the state of the optical disk unit at the time of the recording, the state of the optical disk at the time of the recording, and the like.
In other words, the duration of the laser beam emission for generating the recording pit pattern must be variably controlled depending on the characteristic and recording state of the optical disk. For example, a method proposed in a Japanese Laid-Open Patent Application No. 8-249702 realizes such a variable control by generating write pulses which is subjected to a pulse width control (write strategy). An adjusting width of this pulse width control (write strategy) is described by a ratio with respect to T (an integer multiple of T).
Next, a description will be given of a problem associated with the pulse width control (write strategy) in the optical disk unit employing the CAV recording system.
Conventionally, a writable compact disc (CD) employs the CLV recording system. In the optical disk unit employing the CLV recording system, a channel clock (channel bit) period T which is used as a reference for the recording and reproduction does not change because the linear velocity is constant. Hence, pattern lengths of the individual pit patterns described by sizes of 3T to 11T also do not change. The write pulse width is finely adjusted by the pulse width control (write strategy), so as to optimize the recording quality of the optical disk with respect to the pit patterns. The pulse width control (write strategy) is unaffected by the radial position where the recording operation is carried out.
In the optical disk unit employing the CAV recording system, the angular velocity is constant, and for this reason, the width of 1T must be changed depending on the radial position where the recording operation is carried out. For example, in the case of an 8-times speed optical disk unit employing the CAV recording system, it is possible to use a channel clock equivalent to that of an 8-times speed optical disk unit employing the CLV recording system at the outer periphery of the optical disk, but a channel clock similar to that of a 3-times speed optical disk unit employing the CLV recording system must be used at the inner periphery of the optical disk. For the pulse width of 1T, a difference of 40 ns or greater exists between the channel clocks for the outer periphery and the inner periphery of the optical disk.
Consequently, the pulse width control (write strategy) which takes into consideration the characteristic of the optical disk itself, the state of the optical disk unit at the time of the recording, the state of the optical disk at the time of the recording, and the like, must also be changed depending on the radial position where the recording operation is carried out. The pulse width control (write strategy adjusts the pulse width with respect to each of the pit patterns, such as 3T, 4T, 5T, . . . , and 11T. Hence, in the optical disk unit employing the CAV recording system, the radial position where the recording operation is carried becomes a determining factor also when determining the relationship of the pit pattern length (width) and the adjusting width of the write strategy.
In other words, although the adjusting width of the pulse width adjustment (write strategy) with respect to one pit pattern is not dependent on the radial position where the recording operation is carried out in the case of the optical disk unit employing the. CLV recording system, it is necessary to regularly change a set value of the adjusting width of the write strategy depending on the linear velocity, that is, the radial position where the recording operation is carried out, in the case of the optical disk unit employing the CAV recording system.